1. Technical Field
The present invention is directed to a bladder for a shoe midsole, and in particular, to a bladder having a plurality of distinct chambers, with at least one chamber pressurized to a different pressure than the remaining chambers, and a method for so inflating the bladder.
2. Description of the Prior Art
Bladders used for cushioning shoes are known in the art. Such bladders generally are made of an elastomeric material and are formed so as to have an upper or lower surface enclosing one or more chambers therebetween. The chambers are pressurized above ambient pressure by insertion of a nozzle or needle connected to a fluid pressure source into a fill inlet formed in the bladder. After the chambers are pressurized, the fill inlet is sealed, for example, by welding, and the nozzle is removed. A bladder pressurized in this fashion is disposed during manufacture of a shoe between the outsole and the insole for at least a portion of the extent of the shoe. Thus, the bladder forms all or part of the midsole of the shoe and serves to provide cushioning. If desired, a conventional foam material may be disposed between the outsole and insole at the locations not occupied by the bladder to serve as the cushioning midsole at those locations. Further, the bladder may be partially or totally encapsulated by the foam.
Bladders of this type may be manufactured by the prior art two-film technique in which two separate sheets of elastomeric film are formed having the overall peripheral shape of the bladder. The sheets may be welded together along the periphery to form a bladder having upper, lower and side surfaces, and at predetermined interior areas to give the bladder a preferred configuration, that is, to have chambers of a predetermined shape and size at desired locations. Alternatively, the two sheets may be vacuum-formed to have the preferred configuration and then welded together. In either case, the bladder is formed so as to have one or more fluid inlets through which a needle can be inserted to inflate the various chambers.
Bladders also may be manufactured by the prior art blow-molding technique. A liquified elastomeric material is placed in a mold having the desired overall shape and configuration of the bladder. The mold has an opening at one location through which pressurized air is provided. The pressurized air forces the liquified elastomeric material against the inner surfaces of the mold and causes the material to harden in the mold to form a bladder having the preferred shape and configuration. A sprue appendage is formed at the location of the mold opening and may serve as the fluid fill inlet into which a nozzle is inserted.
Bladders manufactured in this manner are especially useful in providing cushioning in athletic shoes. Different types of athletic activities require different degrees of cushioning at different locations throughout the extent of the shoe. Thus, it desirable to manufacture the bladder with chambers which are isolated from each other at different pressures and which have different enclosed volumes. For two chambers having the same volume, the chamber at the higher pressure will provide more resistance to compression, that is, the higher pressure chamber will be stiffer. Similarly, for two chambers at the same pressure, the chamber with the smaller volume will be stiffer. By manufacturing bladders with distinct chambers enclosing different volumes at desired locations throughout the shoe, and by inflating the chambers to a predetermined pressure, a bladder can be made having a desired stiffness at any location of the shoe. The bladder and thus the shoe can be tuned to a particular activity.
However, in the prior art, inflating the chambers to the predetermined pressure has been difficult when it is desired to inflate one or more chambers to a different pressure than the remaining chambers. For example, in the two-film technique, if it is desired for the bladder to have chambers at different pressures, the bladder must be formed so as to have one or more of the chambers isolated from the remaining chambers. However, in order to allow for inflation of the isolated chamber(s), the bladder must be formed with a separate fill inlet for each chamber(s) which is to be inflated at a given pressure. This complicates the manufacturing process and increases expense. Additionally, since it is desirable to have portions of the bladder exposed after assembly in a shoe, and since the fill inlets are aesthetically unappealing, the use of bladders having more than one fill inlet restricts the design possibilities for the shoe. Further, each fill inlet has a smaller diameter than the chambers and thus provides less cushioning.
Similarly, in the blow-molding technique, if it is desired for the bladders to have chambers at different pressures, the bladders must be formed so as to have one or more of the chambers isolated from the remaining chambers, and with a separate fill inlet for each isolated chamber(s). However, it is difficult to manufacture the bladder so as to have more than one sprue, and thus, with more than one fill inlet. Forming bladders with even two sprues is costly and complicated, and depending upon the desired shape and configuration of the bladder, may not be possible at all. Accordingly, with either prior art technique, forming bladders with chambers at predetermined locations having different levels of pressurization is difficult, expensive and sometimes not possible at all.